Performance monitoring to determine channel presence and per-channel power has been employed using detection of an entire spectrum of channels of an optical signal wherein each channel has had impressed upon it a known dither. The dither can be implemented in many ways, such as amplitude modulation (AM) of a tone or set of tones or a code division multiple access (CDMA) modulation scheme, etc.
In an example using AM detection each channel of a multiplexed optical signal has impressed upon it a unique AM tone and power of each channel of the optical signal is measured through differentiation of the unique AM tones and knowledge of a fixed modulation depth and total optical power. This technique has the benefit of being easy to implement and requires a broadband optical tap and a PIN detector to perform the monitoring function. In addition, one can uniquely identify the source of an optical signal by modulating AMs with wave identification (WID) information. The accuracy of this method in estimating per channel power is limited by effects such as stimulated Raman scattering (SRS) which limits its usage to systems with a small number of spans, small channel counts and low per channel powers.
Modern systems are striving to achieve greater span, higher channel count and require higher channel power, thus limiting the applicability of this approach. Another method is to use an optical spectrum analyzer (OSA) at each monitoring point or a single OSA with an optical switch for all points that are co-located (along the path of the optical signal). This approach provides accurate per-channel power measurements that are not affected by SRS. However, in the past, OSAs have not been equipped with the capability of discerning WID information. In addition, it is usually much more expensive to implement, slower in taking measurements and the optical switch can limit the overall reliability of the OSA function. Furthermore, an OSA takes up more space than a PIN detector and measurements at a plurality of co-location points using one or more OSAs causes fiber routing and handling issues.
Finally, SRS can affect system performance. Consequently, a measurement of the SRS present in a system is therefore needed so that it can be used to control the system to minimize the impact of the SRS.